A vehicle safety system typically incorporates crash sensors and safety devices which are connected to an electronic control unit. The electronic control unit receives signals from the sensors and processes the signals to determine whether the signals indicate a crash situation. If the electronic control unit determines that a crash situation is occurring then the electronic control unit provides an output signal to at least one of the safety devices to activate the safety device.
Many safety devices, such as airbag modules, incorporate a pyrotechnic squib. One output of the electronic control unit is connected to deliver a firing signal to the pyrotechnic squib to activate the squib in the event of a crash situation. The firing signal has suitable characteristics to activate the pyrotechnic squib and is usually in the form of a current of 1.2 A which flows for 2 ms or a current of 1.75 A that flows for 0.5 ms. The current in the firing signal is above a predetermined level which is required to trigger the pyrotechnic squib to activate the safety device. An electronic control unit will usually incorporate a plurality of outputs to drive a plurality of pyrotechnic squibs positioned in different safety devices in a vehicle.
The firing signals which are provided by the electronic control unit are suitable for activating a pyrotechnic squib. However, the firing signals are usually not suitable for triggering other vehicle safety devices which do not incorporate a pyrotechnic squib. Such safety devices could, for instance, include a fuel cut off device or a device for unlocking the doors of a vehicle.
Some electronic control units incorporate a crash output which is provided to trigger vehicle safety devices which do not incorporate a pyrotechnic squib. The crash output provides a signal having an appropriate current and voltage level and an appropriate signal coding to trigger a vehicle safety device. A crash output from an electronic control unit is an effective trigger for a vehicle safety device which does not incorporate a pyrotechnic squib. However, not all electronic control units incorporate a crash output because the crash output is sometimes omitted to reduce costs.
Even if a crash output is provided on an electronic control unit, the crash output can be problematic because there is no established standard that the crash output must follow. Therefore, a system designer must match a particular safety device with a particular crash output in order for the system to function correctly. This minimises the choice available to a system designer and can increase the cost if the designer is required to customise a system to match a particular crash output.
It has been proposed previously to connect an electronic control unit to a vehicle's controller area network (CAN) bus and to connect a safety device to the CAN bus so that the electronic control unit can trigger the safety device via the CAN bus. This enables a safety device that does not incorporate a pyrotechnic squib to be triggered by the electronic control unit. However, there are problems with this arrangement since a CAN bus usually extends throughout a vehicle and can become damaged during a crash situation. The damage may prevent the signal from the electronic control unit being conveyed to the safety device. Furthermore, there is no established standard that activation signals on a CAN bus must follow and so a system designer is limited to using proprietary hardware and software to trigger a safety device via a vehicle's CAN bus.
The present invention seeks to provide an improved drive arrangement for driving a vehicle safety device.